Inside the secret room of Eos Tower, I located the monster disguised as a dollhouse, destroyed it, recovered the pendulum, and gave it to Mark the Toy Soldier. This should get the Ludibrium Clocktower to start working again.
You met Olson the Toy Soldier who's standing guard with Delv on 100th Floor of Eos Tower. Olson told you that a monster that came through a Crack in Time ran off after stealing a pendulum from the Ludibrium Clocktower, and disguised itself as a Doll's House in this room. You must find a slightly different Doll's House among all the Houses inside.